The vertical part of the inverted ‘T’ (right) changes its length. Click on it, the movement will stop, and you can now drag the size until you are satisfied that the vertical and horizontal parts are identical in length. Then pressing the “Show result” button, the length in percent is displayed and the veridical length is overlaid in green.

Comment

Most explanations don’t convince me particularly (but see the new reference below {M&T 2013} for good background information in addition to nice experiments).
Also known as the “Oppel-Kundt illusion”. A nice variant on the bottom right: the dashed lines, indicating height & width of the top hat, indeed span identical distance.